Natural killer (NK) cells are a type of cytotoxic lymphocyte that constitutes a major component of the immune system. NK cells play a major role in the rejection of tumors and cells infected by viruses. The process by which an NK cell recognizes a target cell and delivers a sufficient signal to trigger target lysis is determined by an array of inhibitory and activating receptors on the cell surface. NK discrimination of self from altered self involves inhibitory receptor recognition of MHC-I molecules and non-MHC ligands like CD48 and Clr-1b. NK recognition of infected or damaged cells (altered self) is coordinated through stress induced ligands (e.g., MICA, MICB, Rae1, H60, Mult1) or virally encoded ligands (e.g., m157, hemagluttinin) recognized by various activating receptors, including NKG2D, Ly49H and NKp46/Ncr1.
The process of initial target cell recognition and the recruitment of appropriate downstream signaling molecules to the NK synapse is carefully coordinated in order for the NK cell to effectively kill the target. Although many of the key players in the process are known, the manner in which these disparate steps and pathways are coordinated is less well understood. NK activating receptors, such as NKG2D and Ly49H, upon ligand engagement are able to bind DAP10 or DAP12 molecules that contain an ITAM or YxxM motif. This then allows for the recruitment of various effectors of cell signaling, including the Src and Syk related protein tyrosine kinases that subsequently lead to the activation of more distal effector pathways such as the PI3K and MAP/ERK pathways. Inhibitory receptors that engage self-ligands can oppose activation of these pathways through the recruitment of various SH2 domain-containing phosphatases to their ITIM. These include SHP1 and SHP2, which are responsible for the removal of tyrosine phosphates and the inositol phosphatase SHIP, which is responsible for the removal of the 5′ phosphate from PI, P3.
2B4 is a member of the SLAM family of receptors. It functions through the recognition of another SLAM family member, CD48, which is ubiquitously expressed on cells of the hematopoietic system. 2B4 has a complex role in NK cell function and physiology that remains an active area of investigation. Depending on the context 2B4 has been shown to act as both an inhibitory and activating receptor. This is likely due, at least in part, to the ability of 2B4 to differentially recruit various downstream effectors of cell signaling. Under different signaling contexts and in different species 2B4 can recruit SAP, EAT-2, FynT, SHP1, PI3K and SHIP. How the differential recruitment of these signaling entities is controlled is not completely understood. However, which molecules are recruited and thus which signal is propagated following CD48 engagement may be influenced by the ratio of 2B4 isoforms expressed in the NK cell. Two 2B4 isoforms have been identified in mice, short (2B4S) and long (2B4L), that were proposed to have activating and inhibitory signaling capacities, respectively. Although the exact function of these two isoforms remains to be defined, it is feasible that the different intracellular domains within these isoforms could recruit different effectors of cell signaling. 2B4 could also mediate different signaling outcomes through changes in the availability or recruitment of different signaling molecules. For instance, it has been shown that there are diminished levels of the SAP protein in immature human NK cells. The lack of this key activating molecule in the cell appears to lock 2B4 into an inhibitory signaling mode. In other SLAM family members, namely CD150, there is evidence that the presence or absence of SAP can regulate the binding of both SHP1 and SHIP to the immunoreceptor based tyrosine switch motifs (ITSM) of this receptor.